Iontophoresis, according to Dorland's Illustrated Medical Dictionary, is defined to be "the introduction, by means of electric current, of ions of soluble salts into the tissues of the body for therapeutic purposes." Iontophoretic devices have been known since the early 1900's. British patent specification No. 410,009 (1934) describes an iontophoretic device which overcame one of the disadvantages of such early devices known to the art at that time, namely the requirement of a special low tension (low voltage) source of current which meant that the patient needed to be immobilized near such source. The device of that British specification was made by forming a galvanic cell from the electrodes and the material containing the medicament or drug to be transdermally delivered. The galvanic cell produced the current necessary for iontophoretically delivering the medicament. This ambulatory device thus permitted iontophoretic drug delivery with substantially less interference with the patient's daily activities.
More recently, a number of United States patents have issued in the iontophoresis field, indicating a renewed interest in this mode of drug delivery. For example, Vernon et al U.S. Pat. No. 3,991,755; Jacobsen et al U.S. Pat. No. 4,141,359; Wilson U.S. Pat. No. 4,398,545; and Jacobsen U.S. Pat. No. 4,250,878 disclose examples of iontophoretic devices and some applications thereof. The iontophoresis process has been found to be useful in the transdermal administration of medicaments or drugs including lidocaine hydrochloride, hydrocortisone, fluoride, penicillin, dexamethasone sodium phosphate and many other drugs. Perhaps the most common use of iontophoresis is in diagnosing cystic fibrosis by delivering pilocarpine salts iontophoretically. The pilocarpine stimulates sweat production; the sweat is collected and analyzed for its chloride content to detect the presence of the disease.
In presently known iontophoretic devices, at least two electrodes are used. Both of these electrodes are disposed so as to be in intimate electrical contact with some portion of the skin of the body. One electrode, called the active or donor electrode, is the electrode from which the ionic substance, medicament, drug precursor or drug is delivered into the body by iontophoresis. The other electrode, called the counter or return electrode, serves to close the electrical circuit through the body. In conjunction with the patient's skin contacted by the electrodes, the circuit is completed by connection of the electrodes to a source of electrical energy, e.g., a battery. For example, if the ionic substance to be driven into the body is positively charged, then the positive electrode (the anode) will be the active electrode and the negative electrode (the cathode) will serve to complete the circuit. If the ionic substance to be delivered is negatively charged, then the negative electrode will be the active electrode and the positive electrode will be the counter electrode.
Alternatively, both the anode and cathode may be used to deliver drugs of opposite charge into the body. In such a case, both electrodes are considered to be active or donor electrodes. For example, the positive electrode (the anode) can drive a positively charged ionic substance into the body while the negative electrode (the cathode) can drive a negatively charged ionic substance into the body.
It is also known that iontophoretic delivery devices can be used to deliver an uncharged drug or agent into the body. This is accomplished by a process called electroosmosis. Electroosmosis is the volume flow of a liquid (e.g., a liquid containing the uncharged drug or agent) through the skin induced by the presence of an electric field imposed across the skin.
Furthermore, existing iontophoresis devices generally require a reservoir or source of the ionized or ionizable species (or a precursor of such species) which is to be iontophoretically delivered or introduced into the body. Examples of such reservoirs or sources of ionized or ionizable species include a pouch as described in the previously mentioned Jacobsen U.S. Pat. No. 4,250,878, or a pre-formed gel body as disclosed in Webster U.S. Pat. No. 4,382,529. Such drug reservoirs are electrically connected to the anode or the cathode of an iontophoresis device to provide a fixed or renewable source of one or more desired species.
Recently, the transdermal delivery of peptides and proteins, including genetically engineered proteins, by iontophoresis has received increasing attention. Generally speaking, peptides and proteins being considered for transdermal or transmucosal delivery have a molecular weight ranging between about 500 to 40,000 daltons. These high molecular weight substances are too large to passively diffuse through skin at therapeutically effective levels. Since many peptides and proteins carry either a net positive or net negative charge and because of their inability to passively diffuse through skin, they are considered likely candidates for iontophoretic delivery.
In particular, iontophoresis is being considered for long term delivery (i.e., delivery for periods of longer than 24 hours) of a number of drugs, including peptides (e.g., insulin) and proteins. As the length of delivery increases there is a need to develop small unobtrusive iontophoretic delivery devices which can be easily worn on the skin under clothing. One example of a small iontophoretic delivery device designed to be worn on the skin is disclosed in U.S. Pat. 4,474,570. Devices of this type are powered by small low voltage batteries. In addition to the need for developing smaller iontophoretic delivery devices, there is a need to reduce the cost of these devices in order to make them more competitive with conventional forms of therapy such as pills and subcutaneous injections. One method of reducing cost is to use even lower voltage power sources. Unfortunately, as the power source voltage decreases, the drug delivery rate also decreases. Thus, there has been a need for a method of improving the performance characteristics, such as the amount of drug delivered per unit of power, of iontophoretic delivery devices to enable the use of inexpensive low-voltage power sources.
One method of increasing the rate at which drug is delivered from a transdermal iontophoretic drug delivery device is to apply the device on a skin site having optimum drug transport characteristics. For example, Abramson and Gorin in J. Phys. Chem., Vol. 44, pp 1094-1102 (1940); Burnette and Marrero, J. Pharm. Sci., Vol. 75, pp. 738-743 (1986); and Burnette and Ongpipattanakul, J. Pharm. Sci., Vol. 77, pp. 132-137 (1988) have all shown that during iontophoresis, ions are preferentially transported transdermally through shunt pathways in the skin, such as sweat ducts and hair follicles. The face and scalp have the highest density of hair follicles in humans (see Bloom and Fawcett, A Textbook of Histology, 10th ed., W.B. Saunders Co., p 594). Abramson and Gorin have further shown that iontophoretic transport takes place mainly through sweat duct skin pores rather than hair follicle skin pores. Based on this transport mechanism, one would expect that skin sites having a high density of sweat ducts and hair follicles would be preferred sites for iontophoresis. Table 1, taken from Rothman's: Physiology and Biochemistry of the Skin, Chicago, University Press, p 158 (1971), shows the distribution of sweat ducts in human skin.
TABLE 1 ______________________________________ Distribution of Sweat Ducts in Human Skin No. per cm.sup.2 ______________________________________ Palms 424 Soles 417 Dorsa of Hands 231 Forehead 195 Chest and Abdomen 176 Forearm Flexor aspect 174 Extensor aspect 169 Dorsa of Feet 143 Thigh and Leg Medical aspect 89 Lateral aspect 86 Cheek 85 Nape of Neck 65 Back and Buttocks 65 ______________________________________
Based on this data and on the known mechanism for iontophoretic drug delivery one would expect that the preferred delivery sites would be the head, hands and feet. Surprisingly, the present invention provides a method for increasing the rate at which a drug or other beneficial agent is delivered through intact human skin by selecting a skin site other than that having the highest density of sweat ducts.